1. Field of the Invention
This invention relates to a nozzle in an apparatus for the formation, on a ribbon of hot glass, of a coating layer obtained from gas, such as gases of metal carbonyls or of volatile hydrogenated metal compounds that can decompose on touching the hot glass, for example silanes and, particularly, monosilane.
2. Description of the Related Art
From patent FR 2 314 152, a nozzle of this type is known through which a coating gas is sent over the face to be coated of the ribbon of glass being displaced, the coating gas emitted by a distributor which extends crosswise to the ribbon and to the direction of movement of the latter, so that the gas flows out parallel to the surface of the glass, under laminar flow conditions and with a uniform flow rate over the entire width of the ribbon.
For this purpose, the nozzle comprises a gas injection device, a central shaped block, an upstream lateral shaped projection and a downstream lateral shaped projection placed on both sides of the central block so as to offer to the gas arriving from the injection device a flow path along a U-shaped guide channel which extends between the upstream lateral projection and the central block, between the lower face of the central block and the ribbon of glass, and between the central block and the downstream projection and a gas suction device provided at the outlet of the channel between the central block and the downstream projection.
The terms upstream, central and downstream refer to the direction of movement of the ribbon of glass.
The upstream projection and the downstream projection are provided with flat lower faces which extend parallel to the surface of the glass and at a slight distance from this surface (about 1 mm), to minimize the escape of gas between these faces and the glass.
This nozzle operates well when it is desired to deposit on the ribbon of glass a coating of small thickness, for example less than 60 nanometers and/or involving low flow rates of gas, for example on the order of 100 1/min on a nozzle about 3.30 m wide. Actually, for these thicknesses, the gas current that is in the horizontal part of the channel flows out at a speed less than that of the ribbon. This current is, thereby, entirely entrained downstream by the ribbon of glass, so that no escape of gas or clogging on the upstream projection occurs. Also, the escape that can occur under the downstream projection is sufficiently small so that the production periods are long in comparison with the cleaning periods. The waste of glass during cleaning periods is therefore relatively limited.
On the other hand, when it is desired to deposit on the glass a thicker coating layer, for example on the order of 80 nanometers, and/or involving greater gas flow rates, for example on the order of at least 400 1/min on a nozzle about 3.30 m wide, the speed of passage of the gas can clearly be greater than that of the ribbon of glass. But this increase of speed creates the following drawbacks:
The length of the horizontal part of the channel of U-shaped section must be elongated in the same proportion as the speeds, so that the contact time of the gas with the glass is sufficient to both assure its decomposition and to obtain the desired layer thickness.
Since the speed of the gas is greater than the ribbon of glass, the escape of gas under the downstream projection becomes considerable and gas also begins to escape under the upstream projection. The lower faces of these projections quickly clog with deposits of coating material, which creates gas streams where the flow speeds are higher, and is reflected by a nonhomogeneous deposit in the crosswise direction of the ribbon of glass. The latter is then covered with a layer of material whose thickness (and therefore color), luminous reflectance and light transmittance vary crosswise,
When the clogging becomes too great, particles of coating can touch the glass and leave marks.
The deposits which grow over time require a periodic cleaning of the nozzle, thereby causing a waste of glass since the glass production installation continues to deliver during the cleaning.
The nonrecovered escaped gas pollutes the atmosphere of the float bath of the glass when such a nozzle is installed on the inside of a float chamber for the production of the glass.